The present invention relates to integrated circuits which include oscillators.
The behavior of integrated circuits generally tends to be somewhat temperature-dependent. For example, normal logic elements, in a CMOS integrated circuit, will normally run somewhat slower at higher temperatures. In some contexts, this normal temperature-dependence can be very undesirable.
For example, DRAMs (which are the most widely used type of semiconductor memory) require regular refreshing. At higher temperatures, the leakage current of the DRAM cells becomes larger, and therefore it is very important that the frequency of refresh operations not be decreased. If conventional logic circuits were used in the timing circuit which sets the refresh frequency, the refresh frequency might decrease with temperature, which would be undesirable.
Conventional timing circuits will typically use resistor-capacitor combinations to define a time delay. However, the polysilicon resistors most commonly used in integrated circuits will typically have a resistance which increases with temperature. (This is conventionally expressed by stating that the resistor has a positive TCR, or positive temperature coefficient of resistance.) Since the resistance increases, the time delay will also increase. Many attempts have been made to compensate such temperaturedependence. However, many of the previous attempts use bipolar devices, or use transistors operating in an analog mode to control current, or use current-mirror techniques. All of these approaches are unsuitable for very low-power operation.
The present invention has the advantage of providing a very simple circuit which avoids increased delay at higher temperature.
According to this innovative teaching, there is provided an oscillator with reduced temperature-dependence. A time-delay circuit uses both a pull-up resistor and a pull-down resistor, of two different types, to charge a capacitor. The material with the smallest available thermal coefficient of resistance (TCR) is used for the pull-up resistor. Another material, with a larger positive TCR, is used for the pull-down resistor.
This permits an oscillator to be constructed which behaves as if it were constructed using zero-TCR or negative-TCR resistors, even in an integrated circuit process where such elements are not available.
A further innovative teaching is that the oscillator preferably includes two separate time-delay circuits, configured so that the frequency of the oscillator is dependent on the charging delay of each of the time-delay circuits, but is independent of discharging delay. This feature is not only advantageous of itself, but also combines advantageously with the use of pull-up and pull-down resistances having different TCRs.